In a surface light source device of a conventional large-sized liquid crystal display device, a large number of cold-cathode tubes have been arranged immediately under a liquid crystal panel, and these cold-cathode tubes have been used along with members such as a diffusion panel and a reflection sheet. In recent years, the light emitting diode has been used as a light source of the surface light source device. The light emitting diode has recently been improved in efficiency, and expected as a light source with small consumption power to replace a fluorescent. Further, as for the light source for a liquid crystal display device, the brightness of the light emitting diode can be controlled in accordance with a picture, thereby to reduce power consumption of the liquid crystal display device.
In the surface light source device using the light emitting diode of the liquid crystal display device as the light source, a large number of light emitting diodes are arranged in place of the cold-cathode tubes. Although the use of the large number of light emitting diodes can give uniform brightness on the surface of the surface light source device, there has been a problem in that the device cannot be made inexpensive due to the need for the large number of light emitting diodes. An attempt has been made to increase an output of one light emitting diode to reduce the number of light emitting diodes to be used, and for example in Patent Document 1, there is proposed a lens with which a uniform surface light source is obtained even by means of a small number of light emitting diodes.
In order to obtain a uniform light source by means of the small number of light emitting diodes, it is necessary to increase an illuminated area illuminated by one light emitting diode. That is, it is necessary to expand light from the light emitting diode, to widen the directivity thereof. Therefore, in Patent Document 1, a lens having a circular shape in a plan view, which controls directivity of a chip-like light emitting diode, is arranged above the light emitting diode. This lens is shaped such that a portion in the vicinity of an optical axis on a light exit surface that emits light is a depressed surface, and a portion on the outside thereof is a projected surface continued with the depressed surface.
The light emitting diode emits the largest amount of light in a direction to the front surface of the chip of the light emitting diode. In the lens disclosed in Patent Document 1, light headed in the direction from the light source to the front surface is diffused by refraction on the depressed surface in the vicinity of the optical axis. This can suppress an illuminance in the vicinity of the optical axis on a surface to be irradiated, so as to give an expanded illuminance distribution.